Traditional honing feed systems can be classified into two types: 1) Force controlled, and 2) Rate controlled. In a force controlled system, a constant or controlled force is applied to the feed rod/wedge of the honing tool. The force can be applied by a spring, a cylinder, or other means. A measurement system or a mechanical trigger detects when the wedge has reached a point that is either known or inferred to be the finished bore size. In a rate controlled feed system, a motor, typically controlled by feedback from an encoder, moves the feed rod/wedge at a constant or controlled rate. The bore size is inferred from the encoder count and can be calibrated or compensated for through an interactive user interface.
Each type of feed system has is own strengths and limitations. In a force controlled feed system one advantage is speed. Fast, (nearly instantaneous) expansion of honing stones to the point of contact with the workpiece bore and similarly fast retraction at the end of the honing cycle are possible. In constant force systems, the wedge and feed system elasticity does not affect final bore size. There is no tool breakage or excessive abrasive wear due to excessive feed forces. And, workpieces with less stock to be removed will be honed faster, i.e. no time is wasted while the honing stones expand at a relatively slow rate (selected for cutting) through the entire range of the maximum anticipated stock removal. Disadvantages include that cycle time cannot be controlled, i.e. stones that glaze will hone with increasingly longer cycle times. And, abrasives make rapid contact with rough or out of round bores causing tool or fixture damage and/or wearing abrasive stones excessively.
Advantages of a rate controlled feed system include electronic control of size, and electronic display of feed position during cycle without a separate measuring system. And, honing cycle time will be consistent and unaffected by changes in abrasive condition. Disadvantages include that feed rod force is unknown. Feed forces can reach levels that endanger the tool, fixture, or operator. Variations in the pre-process bore size will result in either wasted time or dangerous crash conditions. The honing process must start with the honing tool at some initial size. This position must be set for some point slightly smaller than the smallest anticipated pre-process bore size. Honing a workpiece with a larger initial bore size must therefore include some wasted “air-cutting” time. Any workpiece with an initial bore size that is smaller than the initial tool size will be impacted violently as the abrasive feeds into the bore with full force during the tool's rapid expansion to the starting size. Such impact is likely to the damage tool or the workpiece. And, since the force in the feed rod/wedge is unknown, the elasticity of those elements as well as the elasticity of the entire system introduces an error when inferring bore size from encoder position.
In all types of honing feed systems it is desirable that the feed position (i.e. position of the abrasive stones) be known during the honing process. If the honing system does not include some in-process bore measuring means, then knowing the feed position accurately is essential for determining when the desired final bore size has been reached. Most honing machines use some type of encoder or other position transducer on the feed system to infer the feed position.
For each honing application, optimum performance (as determined by bore quality and cost per bore) will require the honing tool to operate within some limits of feed force and feed rate. Furthermore it is possible that the optimum values of those parameters may be different at different stages in the honing cycle. It is not possible to exactly control both feed force and feed rate. The many variables affecting honing performance will cause one of these two parameters to vary any time the other is controlled exactly. However, there are significant advantages to a feed system that constantly monitors the uncontrolled parameter and then uses that information to adjust the controlled parameter, to change the method of control, or to more accurately determine the position of the abrasive stones.
Some hybrid systems already exist, but they fall short of the full control of the proposed invention as described below:
Reference in this regard, U.S. Pat. No. 3,849,940 (Yoshino et al., Honing Machine) which describes a feed system that contains both a constant force and a constant rate system mechanically coupled in such a way that the faster of the two systems will control the expansion of the honing stones. However, if the constant rate system is in control, then there is no means to measure feed force or to correct bore errors caused by variances in feed force. Also, it is not possible to select the slower system when it is desirable to do so, e.g. to improve bore geometry at the end of the honing cycle.
U.S. Pat. No. 4,187,644 (Fitzpatrick, Dual Feed Apparatus for Multiple Spindle Honing Machine) describes a feed system where a cylinder (constant force system) expands stones to the point where they contact the workpiece bore and then the feed control switches to a constant rate mechanism. However, this system includes no means to measure feed force or to correct bore errors caused by variances in feed force. Also, it is not possible to select the controlled force system other than for the initial rapid expansion of the stones.
U.S. Pat. No. 4,397,658 (Vanderwal, Feed Control For Honing or Like Machines) describes an oil damper device to provide a slower initial feed rate or even a constant feed rate for the entire honing cycle. However, this includes no means to measure feed force or to correct bore errors caused by variances in feed force.
U.S. Pat. No. 4,679,357 (Richter et al., Method and Apparatus for Displacing a Honing Tool) describes a feed system where a low value torque limit is imposed on a feed motor control so that stones may feed initially very fast up to the point of contact with the bore, and thereafter a higher torque limit is allowed for honing. The torque limit of the motor is roughly equivalent to a limit on feed force, although mechanical inefficiencies limit the accuracy of using of torque limits as feed force limits. This system also does not include a means to measure feed force or to correct bore errors caused by variances in feed force. There also appears to be no means to control the honing feed to a desired feed force apart from merely preventing the force from exceeding some limit.
European Pat. No. 0081383 (Fox, Improvements Related to Honing) claims a control system that uses feedback from a means for monitoring feed position and velocity and a means for monitoring feed force. However, the details of the patent describe only a hydraulic feed system with a position encoder. In such a system, feed force is inferred by measurement of hydraulic pressure and subject to errors such as that induced from frictional forces between the hydraulic piston and its bore. Although the patent refers to means for monitoring force and position, the use of an electronic load cell to directly measure feed force is not mentioned.
European Pat. No. EP 0 575 675 B1 (Grimm, et al, Method and Machine for Finishing a Bore in a Work Piece) uses a feed force measuring device but only for the purpose of determining the target end point (final encoder position) for the honing process by expanding the honing tool into a size-calibrated ring with a feed force equivalent to that measured on the previously finished workpiece. In a limited way this compensates for errors caused by the elasticity of the workpiece and the feed system components, but as the compensation is a static correction based on force measurements in the previous workpiece, it describes no means to dynamically correct for variations encountered with the workpiece currently being honed. It relies on the assumption that every workpiece is virtually identical to the previous workpiece in regards to hardness and the amount of material to be removed. However, in most applications, this assumption cannot be made reliably. Also, this method makes no suggestion that honing feed force can be controlled throughout the honing cycle.
In all of the above-referenced prior art patents there appears to be no method to dynamically correct in real time for errors in bore size inference that arise due to variations in the feed force. Also, none of the referenced prior art patents gives the honing machine user the ability to choose between rate controlled mode and force controlled mode or to program a honing cycle to switch between the two modes in a manner that could optimize performance.
Accordingly, what is sought is a feed system for a honing machine which provides a capability to dynamically correct in real time errors in bore size inferred arising from variations in feed force, provides a user the ability to choose between rate and force controlled honing modes, and which overcomes one or more of the disadvantages and shortcomings of the prior art systems set forth above.